The present invention relates to synthetic bone implants and methods for installing same in a living body, and more particularly to synthetic implants which induce osteogenesis in vivo.
Trauma, surgical or orthopaedic procedures, congenital deformities, or degenerative conditions, can result in the loss or absence of natural bone tissue which often requires prosthetic operations in order to replace the missing bone tissue.
Bone is a natural composite material composed of a matrix of organic (viz. collagen) and inorganic (viz., apatite) constituents. In the United States alone, is estimated that well over 1,000,000 patients annually require bone augmentation, and, of these, less than half receive human donor materials. As to the latter source of implant bone material, allograft materials are limited in use, tend to have unpredictable, non-reproducible mechanical properties, and have potential for disease transmission. On the other hand, the removal of autologous bone from the epihyses and metaphyses involving the thigh bone and the tibia, or the iliac crest or rib of a patient for grafting purposes requires that the patient endure an additional surgical procedure and the size and shaping of bone grafts taken in this manner may be inadequate for the correction needed.
As artificial bone implant materials, metals and ceramics have been used to replace missing or diseased portions of bone. However, these materials have several disadvantages when implanted in the human body. Ceramic materials tend to embrittle and release powder generated by abrasion into the surrounding biological tissues. Additionally, composites of ceramic or metals with polymers have been proposed for bone implants but the strengths and/or resistance to embrittlement are not completely satisfactory. Non-porous, solid phase implant materials based on mixtures of polymer and apatite used for replacing hard tissue have been described, for example, in U.S. Pat. No. 5,092,890.
As to metal implants, the elastic modulus of metallic implants (e.g., titanium and chrome) generally are 8 to 16 times greater than that of bone resulting in stress shielding across the implant-cement-bone. This leaves the bone virtually inactive and unstressed resulting in re-sorption, thus enlarging the cavity, and susceptible to infectious reactions. Also, metals ions can elute from a metal implant to cause adverse effects upon surrounding biological tissues. Additionally, by comparison, natural bone is porous, thereby allowing for bone ingrowth, soft tissue compatibility, internal capillary growth, fluid flow, and so forth, which are attributes not possible with metals.
As can be appreciated from the above, an artificial bone replacement material has been in demand that has mechanical properties and ingrowth inducing properties that mimic natural bone. However, conventional bone implants that appeared to promote certain bone ingrowth merely become infiltrated with bone from direct conduction of natural bone present at the interface of the natural bone and bone implant.
Porous synthetic bone matrices have been proposed which are designed to be biodegradable, bioresorbable or reactive, such as described, for example, in U.S. Pat. Nos. 5,645,591, 5,522,895, 5,366,508, 5,676,699, 5,686,091, 5,071,436, 5,683,461 and 5,683,459. Temporary implant materials are not suitable for all cases where bone replacement is needed in traumatized bone tissue. More permanent bone implants having some porosity are described, for example, in U.S. Pat. Nos. 5,152,791, 5,356,436 and 5,192,325, which relate to porous ceramic-based implant materials or coatings therefor, while U.S. Pat. Nos. 5,684,061, 5,650,108, and 4,863,974 relate to bone replacement materials involving porous polymeric matrices. Of these, in U.S. Pat. No. 5,684,061, bone induction was reported for the described implant material, preferably a porous membrane formed of a copolymer of vinylidene fluoride and trifluoroethylene, for relatively small pore sizes, e.g., 0.6 and 5 .mu.m but not for a larger pore size of 30 .mu.m. Relatively large pore sizes generally can be expected to be needed to promote viable capillary and tissue ingrowth to sustain biological growth in many bone implant implementations. U.S. Pat. No. 5,650,108 describes a porous bone replacement and bone cement material formed of a solid polymer of acrylic or methacrylic acid, a liquid component of an acrylic monomer and coarse biocompatible particle granules of plastics or inorganic solids. The density and porosity of the bone cement are controlled by the choice of geometry of the solid granule additive. U.S. Pat. No. 4,863,974 describes bone growth material with a void volume of greater than 70% for filling defects in bone made of expanded polytetrafluoroethylene and calcium phosphate. Very high void volumes in a bone implant material could restrict the structural, mechanical capabilities of the implant material.
New, accurate, long-lasting, and efficacious forms of porous synthetic bone are needed that have the ability to assume the mechanical properties and histological characteristics of natural bone tissue including promotion of bone ingrowth.